Composites of nonwoven webs are well known for a wide variety of end uses such as wipes, surgical drapes, surgical gowns and protective apparel applications.
Prior art multi-layer composites of nonwoven webs having water repellency and water vapor permeability properties have been formed from various combinations of nonwoven web layers. One such combination is material in which mats of microfibers, preferably meltblown nonwoven webs, are laminated to one or more webs of continuous filaments, preferably spunbond filaments.
Meltblown polymeric nonwoven webs are produced by heating a polymer resin to form a melt, extruding the melt through a die orifice in a die head, directing a fluid stream, typically air, toward the polymer melt exiting the die orifice to form filaments or fibers that are discontinuous and attenuated, and depositing the fibers onto a collection surface. Bonding of the web to achieve integrity and strength occurs as a separate downstream operation. Such a meltblown process is disclosed in U.S. Pat. No. 3,849,241. Meltblown webs are characterized by their softness, bulk absorbency, and water repellency properties. The filaments of such webs are generally discontinuous and of relatively low diameter.
Spunbond polymeric nonwoven webs can be produced by extruding a multiplicity of continuous thermoplastic polymer strands through a die in a downward direction onto a moving surface where the extruded strands are collected in randomly distributed fashion. The randomly distributed strands are subsequently bonded together by thermobonding or by needlepunching to provide sufficient integrity in a resulting nonwoven web of continuous fibers. One method of producing spunbond nonwoven webs is disclosed in U.S. Pat. No. 4,340,563. Spunbond webs are characterized by a relatively high strength/weight ratio, isotropic strength, high porosity and abrasion resistance properties. Spunbond nonwoven webs are non-uniform in properties such as basis weight. The filaments of those webs are generally substantially continuous and of greater diameter than those of meltblown webs.
A major limitation of many commercially available multi-layer composite laminates of spunbond/meltblown/spunbond (SMS) nonwoven webs is that the spunbond webs are nonuniform in coverage and basis weight. In many applications, attempts are made to compensate for poor fabric aesthetics and limiting physical properties that result from this nonuniformity of coverage and basis weight by using webs having a greater number of filaments and a heavier basis weight than would normally be required by the particular application if the web had a more uniform coverage and basis weight. This, of course, adds to the cost of the composite product and contributes to greater stiffness and other undesirable features.
In view of the limitations of the spunbond nonwoven webs in multi-layer composites, there is a need for improved composite nonwovens and, particularly, those wherein a self-bonded, fibrous nonwoven web material having very uniform basis weight and balanced physical properties is used as least one layer bonded to at least one layer of a microfibrous, nonwoven web to form a multi-layer polymeric nonwoven web composite.
U.S. Pat. No. 4,196,245 discloses laminates of spunbond and melt-blown nonwoven fabrics having liquid strike-through resistance and air permeability.
U.S. Pat. No. 4,041,203 discloses laminates of spunbond and meltblown nonwovens in which the meltblown nonwoven has a softening temperature of about 10.degree. to 40.degree. C. less than the softening temperature of the spunbond nonwoven. The laminates are suggested for applications such as outer wear linings, jackets, rainwear, pillowcases, sleeping and slumber bags and liners.
U.S. Pat. No. 4,374,888 discloses a three-layer laminate having a basis weight of 2.5 to 10 oz/yd.sup.2 in which the outer layes are spunbond nonwovens and the intermediate layer is a melt-blown nonwoven. The outer layers are treated for resistance to ultraviolet radiation degradation and flame retardance.
U.S. Pat. No. 4,436,780 discloses laminates of a meltblown thermoplastic microfiber web having a basis weight in the range of about 17 to 170 g/m.sup.2 having an average diameter in the range of up to about 10 microns and treated with a surfactant and, on both sides of the meltblown web, a relatively low basis weight web having a basis weight in the range of about 7 to 34 g/m.sup.2 comprising generally continuous thermoplastic filaments having an average diameter in excess of about 10 microns wherein the weight ratio of the meltblown web to the combined outer webs is at least about 2 to 1.
U.S. Pat. No. 4,766,029 discloses a three-layer, semi-permeable, nonwoven laminate in which the two exterior layers are spunbond polypropylene having a melt flow of 35 g/10 min and the interior layer is a two-component meltblown layer of polyethylene and polypropylene with the laminate calendered after formation.
U.S. Pat. No. 4,443,513 discloses soft nonwoven webs of entangled fibers or filaments having a pattern of fused bond areas and a stretched, loopy filament configuration outside the patterned bond area including laminates comprising at least one spunbond layer and at least one microfiber layer having an average diameter of less than 10 microns.
U.S. Pat. No. 4,659,609 discloses a layered abrasive web comprising a meltblown layer having a basis weight of about 5 to about 25 g/m.sup.2 and average fiber diameters of at least about 40 micrometers and a spunbond layer thermally bonded to the meltblown layer.
U.S. Pat. No. 4,863,785 discloses a nonwoven composite material with a melt-blown fabric layer sandwiched between two prebonded, spunbonded reinforcing layers, all continuously-bonded together. The spunbonded material requires prebonding and no parameters or methods of measurement for uniform basis weight are identified.
These patents do not disclose the invented composite nonwoven products comprising at least one layer of a meltblown, discontinuous microfiber web and at least one layer of a substantially randomly disposed, substantially continuous thermoplastic filament web having a high degree of basis weight uniformity, nor do they disclose the improved water repellency and retained water vapor permeability and breathability properties of such composites.
As used herein, a nonwoven web having uniform basis weight is taken to mean a nonwoven web which has a Basis Weight Uniformity Index (BWUI) of 1.0.+-.0.05, wherein the BWUI is defined as a ratio of an average unit area basis weight determined on a unit area sample of the web to an average area basis weight determined on an area sample, N times as large as the unit area sample, wherein N is about 12 to about 18, the unit area sample has an area of 1 in.sup.2, and wherein standard deviations of the average unit area basis weight and the average area basis weight are less than 10% and the number of samples is sufficient to obtain average basis weights at a 0.95 confidence interval. For example, for a nonwoven web in which 60 samples of 1 in.sup.2 squares determined to have an average basis weight of 0.993667 oz/yd.sup.2 and a standard deviation (SD) of 0.0671443 (SD of 6.76% of the average) and 60 samples of 16 in.sup.2 squares (N was 16) determined to have an average basis weight of 0.968667 oz/yd.sup.2 and a standard deviation of 0.0493849 (SD of 5.10% of average), the calculated BWUI was 1.026.
It is an object of the present invention to provide a multi-layer composite nonwoven fabrics comprising at least one layer of a self-bonded, fibrous nonwoven web bonded to at least one layer of a microfibrous, nonwoven web.
Another object of the present invention is to provide a multi-layer composite having a basis weight in the range of about 0.5 to about 5.0 oz/yd.sup.2 comprising at least one layer of a self-bonded web having a plurality of substantially randomly disposed, substantially continuous thermoplastic filaments having a basis weight in the range of about 0.1 to about 3.0 oz/yd.sup.2 with a BWUI of 1.0.+-.0.05 and at least one layer of a microfibrous web comprising a plurality of substantially totally discontinuous thermoplastic filaments having a basis weight in the range of about 0.1 to about 2.0 oz/yd.sup.2.
A further object of the present invention is to provide a multi-layer composite nonwoven web having water repellency and air and water vapor permeability properties comprising at least one layer of a self-bonded, fibrous nonwoven web and at least one layer of a microfibrous web wherein the self-bonded webs and the microfibrous webs are each produced from thermoplastic selected from the group consisting of polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, a blend of polypropylene and polybutene and a blend of linear low density polyethylene and polypropylene.
Among the advantages produced by the multi-layered composites of the present invention are improved water repellency and water vapor permeability properties for a given total basis weight. This improvement is achieved due to the very uniform basis weight properties of the self-bonded, fibrous nonwoven webs comprising substantially randomly disposed, substantially continuous polymeric filaments which enable lower basis weight self-bonded webs to be used to provide strength to the composites. Additionally, the use of blends of polypropylene with polybutene and/or linear low density polyethylene provides the multi-layer compounds with a better hand and improved softness.